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Author Topic: Music Man hybrid  (Read 3836 times)

Bill Moore

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Music Man hybrid
« on: May 16, 2013, 05:32:52 PM »
I am no tech, but enjoy tinkering. I recently picked up an 81 Music Man 100RD amplifier that was dead. Replaced the caps/diodes in the HV, and LV power supplies, and retested it with a bulb limiter. Still had a problem, finally figured out that one of the 5KV diodes at the output tubes was shorted. Found out the function was to protect the OT, and eventually settled on some availlable 3KV diodes.
My question is, how does it generate 100W with only 2 6L6 tubes. The PI is a pair of transistors that are biased at 25mv. The center tap of the OT shows 715V, (in high power mode), but only 350V feeds the power tubes. It is loud!
« Last Edit: May 16, 2013, 05:37:51 PM by Bill Moore »

Enzo

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Re: Music Man hybrid
« Reply #1 on: May 16, 2013, 06:10:12 PM »
Hi, good form would be to link us to the schematic you are using.

The center tap is at 700v, which means the power tube plates are at 700v.  The 350v is on the screens.

The power tubes are cathode driven, so any current through the transistors is also coming through the tube.

Bill Moore

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Re: Music Man hybrid
« Reply #2 on: May 16, 2013, 06:18:13 PM »
Sorry, here is the schematic for this model.

Roly

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Re: Music Man hybrid
« Reply #3 on: May 16, 2013, 11:46:35 PM »
Quote from: Bill Moore
how does it generate 100W with only 2 6L6 tubes.

Deep class-B.  The anode dissipation of a pair of 6L6GC's is 50 watts, and the only way you can meet this and still get anything like 100 watts out is to run them practically cut off at idle, resulting in Class-B operation.  While this results in "loud" it also results in some pretty nasty crossover distortion.  Not one of my favorite amps.
If you say theory and practice don't agree you haven't applied enough theory.

Bill Moore

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Re: Music Man hybrid
« Reply #4 on: May 20, 2013, 10:57:13 AM »
Thank you Roly, I found some articles on class B operation.
I reset the bias this weekend, and had a chance to play it for a while. Granted, I'm not trying to fill a 1K seat house, but in my practice room, it sounds sweet. I A/B'd it with my "Twin" which has an EV SRO15, and feel that set clean, there's not much difference in the Sho-Bud's sound.
Like Enzo said, there is approx. 720V on the plates at idle with the power switch in the HI position.

Roly

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Re: Music Man hybrid
« Reply #5 on: May 22, 2013, 02:06:14 PM »
Quote from: Bill Moore
with the power switch in the HI position

My advice is - don't.   Just because you can set the controls for the heart of the sun doesn't make it a good idea (and it's plenty damn loud in the LO position).  :-[
If you say theory and practice don't agree you haven't applied enough theory.

Bill Moore

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Re: Music Man hybrid
« Reply #6 on: May 22, 2013, 03:49:44 PM »
I probably will never need the high power position, but the procedure to set bias, is to use the high setting, with no input. (Of course I had to measure the voltage after Enzo said what it would be)!